“Plasma enhanced chemical vapor deposition” (or PECVD) is a known technique used to form films on various substrates. For example, Felts et al., U.S. Pat. No. 5,224,441, describes an apparatus for rapid plasma deposition. In the plasma enhanced chemical vapor deposition of silicon oxide, a gas stream including components such as a volatilized organosilicon compound, oxygen, and an inert gas such as helium or argon, is sent into an enclosed chamber at reduced pressure and a glow discharge plasma is established from the gas stream or its components. A silicon oxide layer is deposited upon the substrate when it is positioned near the plasma. In such a system, the pressure is typically reduced from atmospheric pressure by a vacuum pumping system. Electrode surfaces are in electrical communication with the gases introduced into the system such that an electrical discharge or plasma is formed. The purpose of this discharge is to excite moieties in the system and cause them to be deposited onto the workpiece or substrate to be coated.
The use of the “hollow cathode effect” is known from published international patent application No. W0 2006/019565 which is owned by the assignee of the present application, in which the internal surface of tubes and pipes are modified by a treatment process in which the workpiece itself forms the deposition chamber. Treatment is effected within the workpiece by applying a biasing voltage between an electrode within the workpiece, or just at the exterior of the workpiece, and the workpiece itself while passing a treatment gas through the workpiece and maintaining the interior of the workpiece at a reduced pressure. The treatment gas contains the element to be deposited or implanted and the pressure is low enough to establish and maintain the “hollow cathode effect” in which the electron mean free path is slightly less than the diameter of the workpiece, thus causing electron oscillation and implantation or deposition of the desired element below or onto the surface of the component itself.
Plasma deposition systems (either PVD or CVD) generally employ a vacuum chamber of fixed size, with the workpiece to be coated placed in the chamber. The chamber is pumped down before a gas is introduced, and a plasma is generated by applying electrical power between electrodes in the chamber. Different size workpieces can be coated externally as long as the part does not exceed the size limitations of the chamber. However, the ability to coat internal surfaces is very limited. Published international patent application No. W0 2006/019565, which is owned by the assignee of the present application, discloses an arrangement which lends itself to the treatment of internal surfaces. In this arrangement, the pipe is sealed at either end to create a sealed volume that can be evacuated and into which can be passed a treatment gas prior to the application of a bias voltage between anodes positioned outside of the pipe and the pipe itself so as to create the plasma necessary for coating. While this arrangement provides a perfectly acceptable system for treating long components in the field, it is not easy to employ it in the treatment of complex shapes or to accommodate components of differing sizes or to accommodate component apertures of different sizes. Additionally, the anodes themselves are subjected to the coating effect and their performance deteriorates with time.